Starting infection

Probably less than 1,000 SARS-CoV-2 viruses entering the body within minutes can start infection, called the “infectious dose.”  If many more than that enter, then that might cause greater numbers of the viruses to multiply in the body, which might cause more severe disease in some people.  If many less than that enter, the immune system can probably fight them off (Karimzadeh et al.; Mandavilli “It’s not whether”).  Spike proteins on the outside of the virus attach to a protein on the outside of some kinds of human cells, called angio-tensin converting enzyme 2 (ACE2).  ACE2 normally regulates other chemicals on the cells, and maintains blood pressure.  The virus also hijacks other chemicals on the outside of human cells to enter the cells, especially transmembrane protease serine 2 (TMPRSS2), which normally helps the cell take proteins apart so the cell can use the amino acid parts to make other proteins.  So, SARS-CoV-2 infects cells which have ACE2 and TMPRSS2, and causes COVID-19 disease.  The nose has a lot of ACE2, with the eyes, lungs, intestines, and other organs having lesser amounts.  So, the infection usually starts in the nose and eyes, spreads through the respiratory system, and then to many organs (Fischetti et al.; Matheson, Lehner; Zhou et al.).